There may be a more "proper" way but this is how I do it. Take the barreled action out of the stock, then get a dremel tool if you have one and evenly take material out of the barrel channel until you can easily slide a dollar bill all the way down the barrel without any resistance. Thats the easiest way. If you dont have a dremel tool you can get a wooden dowel rod and wrap sandpaper around it, but it takes longer